This invention relates to processes arising out of the use of a chromatographic separation step in combination with electrodialysis water splitting that improve process reliability and efficiency, and enable production of a high quality base product.
In an electrodialysis water splitting processes for the production of an acid and base from salt, the feed salt may be purified to remove certain contaminating anions in order to ensure reliable long term operation. Similarly, the product base also has a certain amount of contaminating anions that need to be removed and then, preferably, the solution containing contaminating anions is recycled.
A bipolar membrane based water splitting process is a low cost, low energy route to the production of acids and bases from their salts. A detailed description of the technology, its applications, and limitations can be found in an article by K. N. Mani (xe2x80x9cElectrodialysis water splitting Technologyxe2x80x9d, J. Membrane Sci., 58, (1991) pp. 117-138). With this technology, two or three compartment cells may be used to convert soluble salts such as sodium chloride (NaCl), potassium chloride (KCl), sodium sulfate (Na2SO4) to their corresponding base (sodium hydroxide NaOH; potassium hydroxide, KOH) and acid (hydrochloric acid, HCl; sulfuric acid, H2SO4).
One limitation for the bipolar membrane technology is that there is a certain amount of salt contaminant present in the end product acid and the base. The problem arises out of the non-ideal permselectivity of the membranes as well as possible internal leakage. The contamination of the acid with salt is usually less problematic; since in many processes the acid is internally consumed (e.g., U.S. Pat. No. 4,391,680xe2x80x94Brine acidification for chlor-alkali plants, U.S. Pat. No. 4,504,373xe2x80x94Sodium sulfate conversion in rayon plants, Pending U.S. application Ser. No. 09/223,054xe2x80x94Production of amino acid hydrochloride and caustic via electrodialysis water splitting). However, salt contamination of the caustic co-product (e.g. NaOH, KOH, LiOH) can present a serious problem in many instances, since certain end uses demand high purity. When the caustic is sold commercially, it has to meet the industry specifications on the salt content. One example of a high purity base product is potassium hydroxide, which is sold at 45 wt % strength, typically has only xcx9c50 ppm of KCl present. Contamination of the product with salt also, in effect, represents a loss of raw material.
Known chromatographic methods separate components in a given stream and are practiced on a large scale for certain applications such as dextrose/fructose separation. The use of ion exchange resins to purify solutions using ion retardation, ion exclusion techniques is known. (See Diaion(copyright) Manual of Ion exchange Resins/Adsorbents; Vol. I, II published by Mitsubishi Chemical Corporation). U.S. Pat. No. 4,154,801 outlines the use of composite ion exchange resin bodies to purify alkali metal hydroxide and carbonate solutions. However, these unit operations have not been used to improve the operation or performance of electrodialysis water splitting processes.
U.S. Pat. No. 5,200,046 describes an apparatus and process for producing acid and base of improved purity. The apparatus uses a multi-compartment cell containing two bipolar membranes per cell. A liquid usually comprising water is circulated in the compartment between the two bipolar membranes, so as to trap the transported salt contaminant. While somewhat effective, the process is complex, and has a relatively high capital and operating (i.e. energy) costs.
Additionally, in certain salt conversion applications, there is need for a more thorough pre-treatment of the salt feed solution. For example in the brine acidification application disclosed in U.S. Pat. No. 4,391,680, the feed salt (NaCl) is acidified in a two compartment cation cell employing bipolar and cation membranes so as to obtain an NaCl/HCl solution which is subsequently processed in a chlor-alkali cell to generate chlorine and caustic soda. Unfortunately, integration of the two compartment cation cell with the chlor-alkali cell is made difficult or impossible because of the presence of chlorate (NaClO3), an oxidizing species, in the recycle brine solution. Such oxidizing species have an adverse effect on the hydrocarbon based membranes that are used in the two compartment cation cells.
A need exists for an improved process for acidifying the brine feed to the electrolysis based chlor-alkali process.
There is also a need for a process for producing high purity base product from a salt raw material, as well as a method for recovering and reusing the salt contaminant.
Electrodialysis water splitting processes have been devised by an incorporation of a chromatographic separation step to substantially completely remove the contaminating salt from the feed salt solution and/or the product base solution. The chromatographic separation step uses an amphoteric ion exchange resin. The resin has a slightly different affinity toward the contaminating salt as compared to the affinity for the bulk component (pure salt) in the stream. When the column is subsequently eluted with water, an effective separation of the components is achieved. The separated pure salt stream is forwarded to the electrodialysis water splitter cell for conversion to acid and base. The base product solution (sodium hydroxide, potassium hydroxide) from the water splitter cell is similarly purified by passing through a second chromatographic column also containing an amphoteric resin, so as to obtain an effective separation of the base from the salt (recovered salt). The purified base solution is then concentrated, as needed, for sale. The recovered salt solution is returned to the electrodialysis water splitter cell for conversion into acid and base.
The inventive process is applicable in the chlor-alkali process for acidifying the brine feed to the electrolysis cell. The feed brine is purified by passing it through a chromatographic column containing an amphoteric ion exchange resin, so as to separate out the contaminating sodium chlorate salt (NaClO3). The purified brine is then acidified by processing it through either a two compartment water splitting cell comprising bipolar and cation membranes or a three compartment water splitter comprising bipolar, cation and anion exchange membranes. An equivalent amount of NaOH is produced in the cell as co-product. The chlorate rich effluent from the chromatographic column is combined with the acidified brine from the water splitter and forwarded to the electrolysis cell.
The inventive process is of particular use in producing a high purity caustic from salt. In the process, a salt such as potassium chloride is purified as needed and converted in a three compartment water splitter comprising bipolar, cation and anion exchange membranes using a direct current driving force. The output from the acid loop might comprise dilute HCl or an acidified compound such as lysine hydrochloride (lysine (HCl), obtained by reacting lysine with the HCl that is produced; or with glutamic acid hydrochloride (glutamic (HCl) that is obtained by reacting glutamic acid with the HCl that is produced), as described in my earlier U.S. application Ser. No. 09/223,054, now U.S. Pat. No. 6,110,342.
The output from the base loop is usually about 10-18 wt % KOH, contaminated with 500-8000 ppm KCl. The base solution is processed through a chromatographic column containing an amphoteric ion exchange resin. The resin has a high affinity for the salt, so that when the column is initially eluted with water a pure concentrated product is recovered first, that is then sent to a downstream concentration step. A further elution with water results in the recovery of a stream that is rich in salt. This stream is returned to the water splitter for conversion to acid and base.
The combination of chromatography and water splitting results in an improved brine acidification process. The combination also results in a novel route to the production of a high quality base product, while also permitting a recovery and reprocessing of the salt contaminant.